The present invention relates to pressure transducers and compositions for making pressure transducers and in particular to transducers having a variable resistance layer made from a composition of conductive particulate intermixed with a continuous matrix resistive resin which is applied in a thin layer to a surface and thereafter cured to form the resistance layer.
Pressure responsive transducers are known. For example, U.S. Pat. No. 3,806,471 to Mitchell describes a typical pressure transducer wherein conduction paths through a volume of a thick compressible layer would be increased thereby decreasing the resistance through the layer in response to application of an external force. These volume type pressure transducers are subject to many disadvantages including the size of the transducer and the relatively rapid degeneration in performance with time and use.
An improved pressure transducer relying on variable contacts across the surface of a thin layer is disclosed in Eventoff U.S. Pat. Nos. 4,489,302 and 4,314,227. However, the Eventoff patents taught that the particulate was an insulative or semiconductor particulate which was believed necessary to insure there would be a resistance across the layer and that the resistance would vary in response to pressure. As such, the Eventoff patents taught away from the use of a conductive particulate. However, it has been discovered that a conductive particulate such as iron spheres or germanium can be used to form an effective pressure transducer when the particulate size and spacing in the resistive resin layer is selected, relative to the thickness of the resin layer, to be within certain ranges.
Accordingly, the pressure transducer of the present invention is made by applying a composition comprising a mixture of a resistive resin and a conductive particulate such as iron spheres or germanium. The resistive resin may be a carbon filled resin which conducts but has a resistance to conductance. The composition is disposed on a base ply to form a resistance layer where the mean spacing between conductive particles creating microprotrusion across the surface of the resistance layer, is greater than about five times the diameter of the particulate and the particle diameter is about 1.3 to three times the thickness of the resultant layer of resistive resin.
The present invention results in a pressure transducer which has stable, predictable performance characteristics largely independent of manufacturing conditions and subsequent environmental changes permitting a significant reduction in the cost of manufacture and an cost effective means of meeting customer tolerances and specifications.